The present invention relates to an ND (neutral density) filter including a plastic sheet substrate and a deposited film formed on the substrate, a method for producing the ND filter, and an aperture device including the ND filter.
An aperture device used in a lens optical system such as a still camera and video camera includes an ND filter formed in a sheet to prevent hunting of an aperture blade and low resolution by diffraction at a small diaphragm opening. Lately, a resolution of a camera has been increased and a size of a camera has been decreased. In such cases, when an ND filter having a uniform light transmittance is used, it may be difficult to effectively prevent the diffraction due to a steep change in a light intensity when the filter enters the aperture opening. Accordingly, it has been proposed to provide an ND filter with various light transmittances changed stepwise.
An ND filter is generally disposed to be movable in a lens optical system. Therefore, it is necessary to use a thin and light material for the ND filter. The ND filter is generally formed in a plastic sheet, so that it is easy to form in a desired shape corresponding to an aperture opening. Such ND filters include a filter in which a plastic material containing an organic dye or pigment is formed in a sheet to provide a desired optical characteristic, and a filter in which a film is deposited on a surface of a transparent plastic material to provide a desired optical characteristic.
In particular, the filter having the deposited film exhibits a high anti-reflection effect at a surface of the filter through a combination of deposited layers. Thus, the filter is effective in reducing a ghost in the lens optical system. Further, it is possible to provide a filter having areas with various light transmittances changed stepwise, so that the film can reduce transmittance without a steep change in the transmittance, thereby preventing the diffraction more effectively.
In general, when the ND filter including a substrate sheet with a deposited film thereon is manufactured, a transparent plastic sheet having a predetermined size is placed in a vacuum chamber, and the deposited film is formed on a surface of the sheet with vacuum deposition, ion plating or sputtering. In the deposition process, in order to prevent curling of the plastic sheet, a periphery of the plastic sheet is fixed with a metal plate in the vacuum chamber, or the entire surface of the plastic sheet is fixed with a mask plate having many openings corresponding to the ND filters.
It is known that a plastic sheet is susceptible to heat. Accordingly, a temperature of the vacuum deposition chamber is maintained as low as possible. However, there is a case in which a plastic sheet is heated up to about 120° C. due to heat when an electron gun heats and melts a deposited material or when the plastic sheet is heated to securely adhere the deposited film to obtain a desired light transmittance.
As a conventional filter, Japanese Patent Publication (KOKAI) No. 10-133253 has disclosed an ND filter using a plastic sheet formed of polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). These materials have a relatively low glass transition temperature of about 70° C. As shown in FIG. 8, when a plastic sheet 1 formed of PET or PEN is heated to about 120° C. in a state that the plastic sheet is fixed at a periphery of an opening 3 of a mask plate 2, it is possible to form wrinkles on the plastic sheet 1 along an edge of the opening 3. Such wrinkles may affect the refraction of light when the sheet is used as the ND filter.
In view of problems described above, an object of the present invention is to provide an ND filter including a plastic sheet having a high glass transition temperature so as to reduce deformation and wrinkles of the plastic sheet due to thermal expansion and shrinkage during the deposition process.
Another object of the present invention is to provide a method for producing the ND filter and an aperture device using the ND filter.
Further objects and advantages of the invention will be apparent from the following description of the invention.